Sort ArrayList of custom Objects by property

Question

I read about sorting ArrayLists using a Comparator but in all of the examples people used compareTo which according to some research is a method for Strings   I wanted to sort an ArrayList of custom objects by one of their properties  a Date object  getStartDay     Normally I compare them by item1 getStartDate   before item2 getStartDate    so I was wondering whether I could write something like   public class CustomComparator       public boolean compare Object object1  Object object2            return object1 getStartDate   before object2 getStartDate              public class RandomName               Collections sort Database arrayList  new CustomComparator

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You can Sort using  java 8  yourList sort Comparator comparing Classname  getName     or  yourList stream   forEach a - gt  a getBObjects   sort Comparator comparing Classname  getValue

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Classes that has a natural sort order  a class Number  as an example  should implement the Comparable interface  whilst classes that has no natural sort order  a class Chair  as an example  should be provided with a Comparator  or an anonymous Comparator class    Two examples   public class Number implements Comparable lt Number gt        private int value       public Number int value    this value   value        public int compareTo Number anotherInstance            return this value - anotherInstance value           public class Chair       private int weight      private int height       public Chair int weight  int height            this weight   weight          this height   height               Omitting getters and setters      class ChairWeightComparator implements Comparator lt Chair gt        public int compare Chair chair1  Chair chair2            return chair1 getWeight   - chair2 getWeight            class ChairHeightComparator implements Comparator lt Chair gt        public int compare Chair chair1  Chair chair2            return chair1 getHeight   - chair2 getHeight              Usage   List lt Number gt  numbers   new ArrayList lt Number gt         Collections sort numbers    List lt Chair gt  chairs   new ArrayList lt Chair gt        Sort by weight  Collections sort chairs  new ChairWeightComparator        Sort by height  Collections sort chairs  new ChairHeightComparator         You can also create anonymous comparators     Sort by color  Collections sort chairs  new Comparator lt Chair gt          public int compare Chair chair1  Chair chair2

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Using Java 8 use can define the Comparator in one line using Comparator comparing    Use any of the following way   Option 1   listToBeSorted sort Comparator comparing CustomObject  getStartDate      Option 2   Collections sort listToBeSorted  Comparator comparing CustomObject  getStartDate

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For sorting an ArrayList you could use the following code snippet   Collections sort studList  new Comparator lt Student gt         public int compare Student s1  Student s2            return s1 getFirstName   compareToIgnoreCase s2 getFirstName

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using the java-8 stream api you can sort an ArrayList by    Comparator lt Person gt  birthdayComparator   Comparator comparing Person  getBirthday    List lt Person gt  sortedList   list stream   sorted birthdayComparator  collect toList

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Well if you using Java 8 or older version Here is the Best solution  Collections sort studentList  Comparator comparing Student  getCgpa  reversed   thenComparing Student   getFname  thenComparing Student  getId     In this case  it will first sort with  getCgpa  first and for the second part it will sort with getFname and getId   Which is field into the pojo class

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With this library here you can sort the list of custom objects on multiple columns  The library uses version 8 0 features  Sample is also available there  Here is a sample to do  SortKeys sortKeys   new SortKeys    sortKeys addField  firstName                addField  age   true      This  true  will sort the age descending     Other ways to specify a property to the sorter are          addField  lastName   String class            addField  dob   Date class  true       Instantiate a ListSorter ListSorter listSorter   new ListSorter        Pass the data to sort  listToSort  and the  by keys  to sort  sortKeys  List sortedList    List lt Person gt   listSorter sortList listToSort  sortKeys

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You can use the Bean Comparator to sort on any property in your custom class

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Since technologies appear everyday  the answer will change in the time  I took a look at LambdaJ and seems very interesting   You can try solving these tasks with LambdaJ  You can find it here  http   code google com p lambdaj   Here you have an example   Sort Iterative  List lt Person gt  sortedByAgePersons   new ArrayList lt Person gt  persons   Collections sort sortedByAgePersons  new Comparator lt Person gt              public int compare Person p1  Person p2               return Integer valueOf p1 getAge    compareTo p2 getAge                     Sort with lambda  List lt Person gt  sortedByAgePersons   sort persons  on Person class  getAge        Of course  having this kind of beauty impacts in the performance  an average of 2 times   but can you find a more readable code

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I prefer this process   public class SortUtil           public static  lt T gt  List lt T gt  sort List lt T gt  list  String sortByProperty                    Collections sort list  new BeanComparator sortByProperty                return list           List lt T gt  sortedList   SortUtil lt T gt  sort unsortedList   startDate      If you list of objects has a property called startDate  you call use this over and over   You can even chain them startDate time   This requires your object to be Comparable which means you need a compareTo  equals  and hashCode implementation   Yes  it could be faster    But now you don t have to make a new Comparator for each type of sort   If you can save on dev time and give up on runtime  you might go with this one

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Since Date implements Comparable  it has a compareTo method just like String does  So your custom Comparator could look like this  public class CustomComparator implements Comparator lt MyObject gt         Override     public int compare MyObject o1  MyObject o2            return o1 getStartDate   compareTo o2 getStartDate              The compare   method must return an int  so you couldn t directly return a boolean like you were planning to anyway  Your sorting code would be just about like you wrote  Collections sort Database arrayList  new CustomComparator      A slightly shorter way to write all this  if you don t need to reuse your comparator  is to write it as an inline anonymous class  Collections sort Database arrayList  new Comparator lt MyObject gt           Override     public int compare MyObject o1  MyObject o2            return o1 getStartDate   compareTo o2 getStartDate                 Since java-8 You can now write the last example in a shorter form by using a lambda expression for the Comparator  Collections sort Database arrayList                            o1  o2  - gt  o1 getStartDate   compareTo o2 getStartDate       And List has a sort Comparator  method  so you can shorten this even further  Database arrayList sort  o1  o2  - gt  o1 getStartDate   compareTo o2 getStartDate       This is such a common idiom that there s a built-in method to generate a Comparator for a class with a Comparable key  Database arrayList sort Comparator comparing MyObject  getStartDate     All of these are equivalent forms

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I found most if not all of these answers rely on the underlying class  Object  to implement comparable or to have a helper comparable interface   Not with my solution  The following code lets you compare object s field by knowing their string name  You could easily modify it not to use the name  but then you need to expose it or construct one of the Objects you want to compare against   Collections sort anArrayListOfSomeObjectPerhapsUsersOrSomething  new ReflectiveComparator    new ListComparator  name      public class ReflectiveComparator       public class FieldComparator implements Comparator lt Object gt            private String fieldName           public FieldComparator String fieldName               this fieldName   fieldName                      SuppressWarnings    unchecked    rawtypes              Override         public int compare Object object1  Object object2                try                   Field field   object1 getClass   getDeclaredField fieldName                   field setAccessible true                    Comparable object1FieldValue    Comparable  field get object1                   Comparable object2FieldValue    Comparable  field get object2                    return object1FieldValue compareTo object2FieldValue                catch  Exception e                 return 0                       public class ListComparator implements Comparator lt Object gt            private String fieldName           public ListComparator String fieldName                this fieldName   fieldName                      SuppressWarnings    unchecked    rawtypes              Override         public int compare Object object1  Object object2                try                   Field field   object1 getClass   getDeclaredField fieldName                   field setAccessible true                   Comparable o1FieldValue    Comparable  field get object1                   Comparable o2FieldValue    Comparable  field get object2                    if  o1FieldValue    null   return -1                   if  o2FieldValue    null   return 1                   return o1FieldValue compareTo o2FieldValue                 catch  NoSuchFieldException e                    throw new IllegalStateException  Field doesn t exist   e                 catch  IllegalAccessException e                    throw new IllegalStateException  Field inaccessible   e

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I have tried lots of different solutions available on internet but solution which works for me is available at below link   https   www java67 com 2017 07 how-to-sort-arraylist-of-objects-using html

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From Java 8 and onward we don t have to use Collections sort   directly  List interface has a default sort   method   List lt User gt  users   Arrays asList user1 user2 user3   users sort   u1  u2  - gt     return u1 getFirstName compareTo u2 getFirstName           See http   visvv blogspot in 2016 01 sorting-objects-in-java-8 html

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This code snippets might be useful  If you want to sort an Object in my case I want to sort by VolumeName   public List lt Volume gt  getSortedVolumes   throws SystemException       List lt Volume gt  volumes   VolumeLocalServiceUtil getAllVolumes        Collections sort volumes  new Comparator lt Volume gt              public int compare Volume o1  Volume o2                Volume p1    Volume  o1              Volume p2    Volume  o2              return p1 getVolumeName   compareToIgnoreCase                      p2 getVolumeName                           return volumes      This works  I use it in my jsp

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Java 8 Lambda shortens the sort   Collections sort stdList   o1  o2  - gt  o1 getName   compareTo o2 getName

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Yes  that s possible for instance  in this answer I sort by the property v of the class IndexValue          Sorting by property v using a custom comparator      Arrays sort  array  new Comparator lt IndexValue gt             public int compare  IndexValue a  IndexValue b                return a v - b v                      If you notice here I m creating a anonymous inner class   which is the Java for closures   and passing it directly to the sort method of the class Arrays   Your object may also implement Comparable   that s what String and most of the core libraries in Java does   but that would define the  natural sort order  of the class it self  and doesn t let you plug new ones

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Yes  you can   There are two options with comparing items  the Comparable interface  and the Comparator interface   Both of these interfaces allow for different behavior   Comparable allows you to make the object act like you just described Strings  in fact  String implements Comparable    The second  Comparator  allows you to do what you are asking to do   You would do it like this   Collections sort myArrayList  new MyComparator       That will cause the Collections sort method to use your comparator for it s sorting mechanism   If the objects in the ArrayList implement comparable  you can instead do something like this   Collections sort myArrayList     The Collections class contains a number of these useful  common tools

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With Java 8 you can use a method reference for your comparator   import static java util Comparator comparing   Collections sort list  comparing MyObject  getStartDate

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Function  amp  method reference  The Collections sort method can sort a List using a Comparator you pass  That Comparator can be implemented using the Comparator comparing method where you can pass a method reference as the necessary Function  Fortunately  the actual code is much simpler and shorter than this description   For Java 8   Collections sort list  comparing ClassName  getName      or  Collections sort list  comparing ClassName  getName  reversed       Another way is  Collections sort list  comparing ClassName  getName  Comparator nullsLast Comparator naturalOrder

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JAVA 8 lambda expression  Collections sort studList   Student s1  Student s2  - gt           return s1 getFirstName   compareToIgnoreCase s2 getFirstName           OR  Comparator lt Student gt  c    s1  s2  - gt  s1 firstName compareTo s2 firstName   studList sort c

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your customComparator class must implement java util Comparator in order to be used  it must also overide compare   AND equals    compare   must answer the question   Is object 1 less than  equal to or greater than object 2   full docs  http   java sun com j2se 1 5 0 docs api java util Comparator html

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You can try Guava Ordering   Function lt Item  Date gt  getStartDate   new Function lt Item  Date gt          public Date apply Item item            return item getStartDate              List lt Item gt  orderedItems   Ordering natural   onResultOf getStartDate                             sortedCopy items

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You could also use Springs PropertyComparator if you have just a String property path to the  nested  property you want to sort   List lt SomeObject gt  list        PropertyComparator lt HitWithInfo gt  propertyComparator   new PropertyComparator lt  gt        property nested myProperty   false  true   list sort propertyComparator     The drawback is  that this comparator silently ignores properties which does not exist or are not accessible and handles this as null value for comparison  This means  you should carefully test such a comparator or validate the existence of the property path somehow

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Best easy way with JAVA 8 is for English Alphabetic sort  Class Implementation  public class NewspaperClass implements Comparable lt NewspaperClass gt      public String name       Override    public int compareTo NewspaperClass another          return name compareTo another name            Sort    Collections sort Your List     If you want to sort for alphabet that contains non English characters you can use Locale    Below code use Turkish character sort     Class Implementation  public class NewspaperClass implements Comparator lt NewspaperClass gt       public String name     public Boolean isUserNewspaper false     private Collator trCollator   Collator getInstance new Locale  tr TR            Override    public int compare NewspaperClass lhs  NewspaperClass rhs          trCollator setStrength Collator PRIMARY         return trCollator compare lhs name rhs name            Sort  Collections sort your array list new NewspaperClass

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Your custom class can implement the  Comparable  interface  which requires an implementation of the CompareTo method  In the CompareTo method  you can then define what it means that an object is less than or more than the other object  So in your example  it can look something like this   public class MyCustomClass implements Comparable lt MyCustomClass gt                   Override public int compareTo MyCustomClass a        if this getStartDate   before a getStartDate              return -1       else if a getStartDate   before this getStartDate              return 1       else           return 0            A negative number indicates that this is smaller than the object being compared to  A positive number indicates that this is larger than the compared to object and a Zero means that the objects are equal   You can then use the collections sort myList  to sort your list without having to feed in a comparator  This method also has the advantage of having things sorted automatically if you use a sorted collection data structures like a TreeSet or a TreeMap    You can check this article if you would like to read more about the Comparable interface  disclosure  I am the author      https   nullbeans com the-java-comparable-interface-automatic-sort-of-collections

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import java text ParseException  import java text SimpleDateFormat  import java util ArrayList  import java util Collections  import java util Comparator  import java util Date   public class test    public static class Person       public String name      public int id      public Date hireDate       public Person String iname  int iid  Date ihireDate            name   iname          id   iid          hireDate   ihireDate             public String toString             return name         id         hireDate toString                  Comparator     public static class CompId implements Comparator lt Person gt             Override         public int compare Person arg0  Person arg1                return arg0 id - arg1 id                       public static class CompDate implements Comparator lt Person gt            private int mod   1          public CompDate boolean desc                if  desc  mod  -1                     Override         public int compare Person arg0  Person arg1                return mod arg0 hireDate compareTo arg1 hireDate                      public static void main String   args           TODO Auto-generated method stub     SimpleDateFormat df   new SimpleDateFormat  mm-dd-yyyy        ArrayList lt Person gt  people      people   new ArrayList lt Person gt         try           people add new Person  Joe   92422  df parse  12-12-2010              people add new Person  Joef   24122  df parse  1-12-2010              people add new Person  Joee   24922  df parse  12-2-2010            catch  ParseException e               TODO Auto-generated catch block         e printStackTrace               Collections sort people  new Person CompId         System out println  BY ID        for  Person p   people            System out println p toString                Collections sort people  new Person CompDate false        System out println  BY Date asc        for  Person p   people            System out println p toString               Collections sort people  new Person CompDate true        System out println  BY Date desc        for  Person p   people            System out println p toString

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You can have a look into this presentation hold at the Java Forum in Stuttgart Germany in 2016   Only a few slides use German language  99  of the content is  English based  Java source code  like  someCollection sort    OurCustomComparator      comparing Person  getName       thenComparing Person  getId       where OurCustomComparator is using default methods  and other interesting ideas   As shown  leading to very concise code to pick some getter method for sorting  and super simple chaining  or reversing  of sort criteria    If you are into java8  you find a lot of material there to get you started

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New since 1 8 is a List sort   method instead of using the Collection sort   so you directly call  mylistcontainer sort    Here is a code snippet which demonstrates the List sort   feature   List lt Fruit gt  fruits   new ArrayList lt Fruit gt     fruits add new Fruit  Kiwi   green  40    fruits add new Fruit  Banana   yellow  100    fruits add new Fruit  Apple   mixed green red  120    fruits add new Fruit  Cherry   red  10        a  using an existing compareto   method fruits sort  Fruit f1 Fruit f2  - gt  f1 getFruitName   compareTo f2 getFruitName      System out println  Using String compareTo        fruits     Using String compareTo     Apple is  mixed green red  Banana is  yellow  Cherry is  red  Kiwi is  green      b  Using a comparable class fruits sort  Fruit f1 Fruit f2  - gt  f1 compareTo f2      System out println  Using a Comparable Fruit class  sort by color       fruits      Using a Comparable Fruit class  sort by color    Kiwi is green  Apple is  mixed green red  Cherry is  red  Banana is  yellow    The Fruit class is   public class Fruit implements Comparable lt Fruit gt        private String name      private String color      private int quantity       public Fruit String name String color int quantity        this name   name  this color   color  this quantity   quantity         public String getFruitName     return name                public String getColor     return color          public int getQuantity     return quantity          Override public final int compareTo Fruit f     sorting the color               return this color compareTo f color                   Override public String toString                    return  name     is      color              end of Fruit class

[java] Sort ArrayList of custom Objects by property

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